Correction of defective human visual perception is centered in general upon clinical refraction, an approach based upon optics, physiology and the psychology of perception. Generally, any refractive analysis of human vision has some basis in optics. For example, the treatment of defective vision will analyze the position of focus of the eye which may be displaced from the emmetropic retina under conditions of either myopia or hyperopia. In addition, the eye may be astigmatic, exhibiting different focal aspects for each primary meridian which, in turn, may be oriented anywhere within a 180.degree. aspect. Thus, the clinician is often called upon to approach the optical subject matter of diagnosis by evaluating the dioptric aspects of focal difficiency as they may be related to meridial power variances. The correction of ocular astigmatism is carried out by collapsing the interval of Sturm with cylinder lenses. See the following publication in this regard:
I. "Visual Optics and Refraction-a Clinical Approach" by D. D. Michaels, second edition, C. V. Mosby Company, St. Louis 1980. PA1 II. Linksz, A.: Determination of Axis and Amount of Astigmatic Error by Rotation of Trial Cylinder. Archives of Ophthalmology, October, 1942. PA1 III. "Clinical Refraction" by I. M. Borish, second edition, The Professional Press, Inc., Chicago, Ill. PA1 IV. Jackson, E.: The Astigmic Lens (Crossed Cylinder). Reprinted in Opt. Devel. August, 1932.
Commonly, an opthalmic instrument referred to as a refractor is employed for efficiently carrying out optical analysis. Refractors typically are fashioned comprising right and left batteries, each having an eye position for the patient before which any of a broad variety of disk mounted testing lenses may be positioned. These lenses may be spherical, exhibiting a broad range of powers, or cylindrical, again exhibiting power variations but with respect to alignment along plus and minus axes.
Investigations of techniques for carrying out ophthalmic evaluation of the astigmatic eye have evolved a variety of analytic approaches. Linksz has described a method for determining meridial orientation, i.e. "checking cylinder for axis and amount" by rotating a correcting cylinder before the eye. This method analyzes the eye with a cylinder lens and considers the principle that if two cylinders are juxtaposed such that their axes do not coincide, a set of optical resultants is produced creating new cylindrical powers and axes. By selecting the correcting cylinder as opposite in power to the eye cylinder, it becomes possible to determine the axial orientation of the eye cylinder and the dioptic power aspects thereof by a progressive error evaluation carried out by manipulating the correcting cylinder. See the following publication in this regard:
The rotating cylinder approach to analysis was further developed by the Jackson cross-cylinder technique wherein the above-noted resultant cylinder developed from the utilization of two cylinders off axis is further contrasted against a third sphero-cylindrical combination. For a more detailed discussion of this clinical approach, reference is made to the following publications:
The Jackson cross-cylinder test has been recognized as most beneficial to analysis and has been implemented broadly in ophthalmic refractors. Generally, the test is carried out in two stages. Initially, an objective evaluation is made of the eye, typically employing retinoscopy procedures which may be further modified using an astigmatic chart test to develop a first approximation of cylinder power and axis. Following this first approximation, the resultant data then are used to develop the subjective, cross-cylinder test which is carried out with the refractor to achieve a refined analysis. The test is carried out in both a cylinder axis and cylinder power mode, most practitioners preferring to carry out the former mode initially. Under the test procedure, the patient is seated in a darkened examination room before the refractor and is asked to observe an illuminated distant target. The correcting cylinder axis before an appropriate eye then is manipulated by manually turning an axis control knob which is operated in conjunction with two complementary large surrounding protractor scales. Such manipulation adjusts the position of the axis of the pertinent test cylinder and its orientation is read at the scale in degrees ranging from 0 degrees to 180 degrees. Typically the graduations of the scales are arranged in steps of 5 degrees. It is from this scale that the ultimate axis information is read out for prescription purposes.
Upon the axis control knob being adjusted to the first approximation a cross cylinder, provided as a lens consisting of equal power plus and minus cylinders with their axes 90.degree. apart is positioned at the eye station. This test lens is mounted in its loupe for rotation about a "flip" axis midway between the plus and minus axes. When the lens is flipped, the plus and minus axes change places. For axis mode testing, the cross cylinder also is positioned with respect to the noted first approximation such that its axis is oriented 45.degree. with respect to the correcting cylinder axis. Such aligning procedure is carried out somewhat semi-automatically. Generally, the refractor will carry the cross cylinder lens within a turret which is manually rotated to position the lens before the eye station or tube. The turret carrying the cross-axis lens, also includes a knurled knob arrangement wherein the operator may readily flip the lens between its above-noted positions. To carry out the initial alignment of the cross-axis lens with respect to the axial orientation of the correcting cylinder lens then before the patients eye, a multiple gear assemblage within the turret which is structured such that by turning the axis control knob, the cross-cylinder lens is synchronously manipulated in a manner wherein its minus axis is aligned with the corresponding correcting cross-cylinder axis. To aid the operator, spaced pairs of red and white dots are positioned upon the cross-cylinder lens to respectively show the location of the minus and plus power axes. To move the cross cylinder to its appropriate orientation for an axis mode test, the operator further rotates the cross cylinder to a mechanical detent controlled orientation to effect a 45.degree. displacement. One technique for assuring the operator that the cross cylinder is in a proper orientation for the axis mode check is an observation of the knurled flip knob position as corresponding to the axis of the correcting cylinder. With the above adjustments carried out, the cross cylinder lens is "flipped" from its first position to the alternate transverse position by rotating the knurled knob with the thumb. The patient then is asked which position is better and depending upon the response and assuming testing is carried out with minus cylinder lenses, the correcting cylinder axis knob is manipulated to rotate the correcting cylinder toward the position of the red dots at which vision is improved. These steps are repeated until a final end point is reached such that when the cross cylinder is flipped from one position to the other, the patient's vision is equally blurred. The operator then records the reading of the axis control knob by observing a painted line indicia thereon as it is positioned adjacent to a line of the earlier-described scale. Generally, the operator interpolates the axial orientation in degrees within given 5 degree steps of the scale.
Following the axis mode check, the cross-cylinder lens is rotated by the operator 45.degree. to another mechanical detent controlled position for carrying out a cylinder power mode check. Visual confirmation of the appropriate position for the cross-cylinder lens usually is provided by noting the position of the earlier-described red dots as being parallel to the correcting cylinder minus axis. As the patient monocularly fixates upon the illuminated target, the cross cylinder lens is flipped between alternate positions and the patient is asked, as before, at which position vision is better or worse. If vision is less blurred with the red dots parallel to the minus correcting cylinder axis, correcting minus cylinder power is increased. If vision is better with the red dots perpendicular to the minus correcting cylinder axis, the correcting cylinder power is reduced. Finally, an end-point is obtained wherein correcting cylinder power is correct and the vision of the patient equally is impaired when the cross-cylinder lens is flipped between its alternate positions. With completion of the test in both the cylinder axis and cylinder power modes, the cross-cylinder test procedure is completed. This test is carried out in both modes usually for both eyes of the patient commencing with the right eye.
While industry has endeavored to achieve a high quality of performance for ophthalmic refractors and has striven to minimize the opportunities for operator error in utilizing the devices, improvement in overall diagnostic accuracy on the part of the instruments still is needed. The synchronization of axial orientations between the correcting cylinder lens and the cross-cylinder lens involves a gear train drive which inherently exhibits backlash charactertistics. These backlash characteristics have been observed to generate errors in synchronization between the correcting lens and the cross-cylinder lens typically on the order of 3.degree. and, on occasions, to the extent of about five degrees. The gear train associating these lenses is subjected to operator imposed torques and forces in consequence both of the initial manual maneuvering of the cross-cylinder lens carrying turret into its operative position as well as by the subsequent manipulations of the lens into its 45.degree. detent position for axial mode performance. Commonly, the lubricant within the gear structures dries to alter its viscosity, dirt and lint are "picked up" within the trains and bearing surfaces to engender maneuvering error. Such environmental and use related conditions lead to inaccurate preliminary lens settings. Further, in flipping the cross-cylinder lens by thumb actuation of the knurled knobs from which it is pivotally driven a torque again may be observed to be induced into the gear structured mounting. As the instruments age and are used, such action also can alter the axis position otherwise established by the detent effecting turret alignment. Thus the errors in synchronization are compounded by forces exerted at either end of the synchronizing gear chain, ie. from the axis control knob and from the cross cylinder lens loupe itself. The errors thus occasioned in the typical use of refractors, of course, are translated into prescription error. It may be noted that particularly where cylinder axis power as is required by the patient becomes higher then the criticality of proper axial readout correspondingly becomes elevated.
While the utilization of relatively broad, five degrees increments typically in conjunction with the protractor scale associated with the axis control knob may be appropriate when considering the amount of inherent error due to gear train backlash, the operator further is called upon to read this rather broad incremental scale in the darkened environment of an examination room. This further leads to potential operator error both in the direct readout and in the attempt to interpolate between five degrees spaced indicia. The darkened environment as well as potential operator fatigue also may lead to error in carrying out the proper adjustment of the cross cylinder lens. For example, the operator must recall that the lens is to have one orientation for axis mode performance and must then be rotated to a second 45.degree. displaced orientation for cylinder power mode testing. Where the operator forgets this manipulation or fails to properly read printed indicia or respond to the orientations of the red and white dots on the cross-cylinder lens in the darkened environment, then important error ensues in diagnostic testing.